We report the results of a study of the thermoelectric power (Seebeck effect), the electrical resistivity, X‐ray diffraction, and Raman spectroscopy under high pressure (P) to 15 GPa for binary and tin‐doped lead selenide (PbSe and Pb1–x Snx Se (x = 0.125)). The gathered data were inconsistent with previous models suggesting only one intermediate phase between the ambient NaCl (B1) lattice and a high‐pressure metal CsCl (B2) lattice. Furthermore, the data hint at the existence of two competing intermediate phases. From the thermopower we established that a phase that is characterized by n‐type conductivity is dominating, while the other one – of p‐type conductivity is probably optional. The electrical conductivity in both of the phases is semiconducting. In the X‐ray diffraction studies in one of the three investigated samples of Pb0.875Sn0.125Se we could refine a pattern of the intermediate phase taken at ∼9.5 GPa in the Cmcm space group. This corresponds to the symmetry of earlier proposed structural types of TlI and CrB. The patterns of the intermediate phase in the two other samples show additional strong reflexes that are not related to either the NaCl or the CsCl lattices. This presents evidence of the presence of one more undetermined phase. The Raman spectra of the intermediate phase showed a dependence on a pressure‐transmitting medium (methanol‐ethanol or KCl). The pressure‐driven NaCl → CsCl transition and a pressure dependence of the semiconductor energy gap of PbSe are compared with those of other lead salts, namely, PbTe and PbS (galena). It is surmised that there is no interrelation between the closure of the energy gap at the NaCl phase and the phase transition to the intermediate phase. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The measurements of the magnetization in high steady and pulsed fields together with neutron diffraction measurements on a powder sample and on a single crystal have been performed to study the magnetic state of the Tb 3 Co compound. It has been shown that the modulated antiferromagnetic structure which exists in Tb 3 Co below T N = 82 K transforms to the incommensurate magnetic structure with a strong ferromagnetic component along the c-axis with further cooling below the critical temperature T t ≈ 72 K. The phase transition from the high-temperature to the low-temperature magnetic state at T t is of first order. The incommensurability of the low-temperature magnetic structure of Tb 3 Co is attributed to the non-Kramers character of the Tb 3+ ion in combination with competition between the indirect exchange interaction and the low-symmetry crystal electric field.
We investigated the effects of applied high pressure on thermoelectric, electric, structural, and optical properties of single-crystalline thermoelectrics, Bi2Te3, BixSb2−xTe3 (x = 0.4, 0.5, 0.6), and Bi2Te2.73Se0.27 with the high thermoelectric performance. We established that moderate pressure of about 2–4 GPa can greatly enhance the thermoelectric power factor of all of them. X-ray diffraction and Raman studies on Bi2Te3 and Bi0.5Sb1.5Te3 found anomalies at similar pressures, indicating a link between crystal structure deformation and physical properties. We speculate about possible mechanisms of the power factor enhancement and suppose that pressure/stress tuning can be an effective tool for the optimization of the thermoelectric performance.
In the present paper the results of fitting synchrotron diffraction data are obtained for the intermediate high-pressure phase (9.5 GPa) of the lead selenide based compound Pb(1-x)Sn(x)Se (x = 0.125)-an optoelectronic as well as a thermoelectric material-for two types of lattice symmetries Pnma (space group #62) and Cmcm (space group #63). Both lattice parameters and positions of atoms for the above mentioned structures have been used in calculations of the electron structure of high-pressure phases. The main difference between the electronic properties for Cmcm and Pnma structures established in electronic structure calculations is that in the first one the PbSe compound was found to be a metal, while in the second a small semiconductor gap (E(G) = 0.12 eV) was obtained. Moreover, the forces in the Cmcm structure are an order of magnitude larger than those calculated for the Pnma lattice. In the optimized, Pnma structure within a generalized gradient approximation (GGA), the band gap increases up to E(G) = 0.27 eV. The result coincides with the data on thermoelectric power and electrical resistance data pointing to a semiconductor gap of ∼0.2 eV at ∼9.5 GPa. Thus, the Pmna type of lattice seems to be a preferable version for the intermediate phase compared with the Cmcm one.
A symmetry analysis of possible magnetic structures in crystals with the space group Dih is made for the wave vector of star { k l s } . This analysis allows deciphering the neutron diffraction data on the character of antiferromagnetic ordering in the AF, (below T, = 66 K) and AF, (below T2 = = 99 K) phases. It is shown that the magnetic phase transitions arise mainly from the structural transformations that occur a t T, and T,.Eine Symmetrieanalyse der moglichen magnetischen Strukturen in Kristallen mit der Raumgruppe D& wird fur den Wellenvektor des Sterns {kls} durchgefuhrt. Die Analyse erlaubt die Entzifferung der Neutronenbeugungsdaten in Abhiingigkeit vom Charakter der antiferromagnetischen Ordnung in den Phasen AF, (unterhalb Tl = 66 K) und AF, (unterhalb T, = 99 K). Es wird gezeigt, daB die magnetischen Phasenubergiinge hauptsiichlich aus den strukturellen Ubergiingen herruhren, die bei Tl und T, auftreten.
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